package me.ivps.android.weibo.support.lib;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ListView;

/**
 * 自动滚动 ListView 控件
 * 
 * User: qii Date: 13-3-31
 * https://github.com/android/platform_packages_apps_contacts
 * /blob/master/src/com/android/contacts/widget/AutoScrollListView.java
 * <p/>
 * ref http://cyrilmottier.com/2013/01/09/back-to-top-android-vs-ios/
 */
public class AutoScrollListView extends ListView {
    
    /**
     * Position the element at about 1/3 of the list height
     */
    private static final float PREFERRED_SELECTION_OFFSET_FROM_TOP = 0.33f;
    
    private int mRequestedScrollPosition = -1;
    private boolean mSmoothScrollRequested;
    
    public AutoScrollListView(Context context) {
        super(context);
    }
    
    public AutoScrollListView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    
    public AutoScrollListView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }
    
    /**
     * Brings the specified position to view by optionally performing a
     * jump-scroll maneuver: first it jumps to some position near the one
     * requested and then does a smooth scroll to the requested position. This
     * creates an impression of full smooth scrolling without actually
     * traversing the entire list. If smooth scrolling is not requested,
     * instantly positions the requested item at a preferred offset.
     */
    public void requestPositionToScreen(int position, boolean smoothScroll) {
        mRequestedScrollPosition = position;
        mSmoothScrollRequested = smoothScroll;
        requestLayout();
    }
    
    @Override
    protected void layoutChildren() {
        super.layoutChildren();
        if (mRequestedScrollPosition == -1) {
            return;
        }
        
        final int position = mRequestedScrollPosition;
        mRequestedScrollPosition = -1;
        
        int firstPosition = getFirstVisiblePosition() + 1;
        int lastPosition = getLastVisiblePosition();
        if (position >= firstPosition && position <= lastPosition) {
            return; // Already on screen
        }
        
        final int offset = (int) (getHeight() * PREFERRED_SELECTION_OFFSET_FROM_TOP);
        if (!mSmoothScrollRequested) {
            setSelectionFromTop(position, offset);
            
            // Since we have changed the scrolling position, we need to redo
            // child layout
            // Calling "requestLayout" in the middle of a layout pass has no
            // effect,
            // so we call layoutChildren explicitly
            super.layoutChildren();
            
        }
        else {
            // We will first position the list a couple of screens before or
            // after
            // the new selection and then scroll smoothly to it.
            int twoScreens = (lastPosition - firstPosition) * 2;
            int preliminaryPosition;
            if (position < firstPosition) {
                preliminaryPosition = position + twoScreens;
                if (preliminaryPosition >= getCount()) {
                    preliminaryPosition = getCount() - 1;
                }
                if (preliminaryPosition < firstPosition) {
                    setSelection(preliminaryPosition);
                    super.layoutChildren();
                }
            }
            else {
                preliminaryPosition = position - twoScreens;
                if (preliminaryPosition < 0) {
                    preliminaryPosition = 0;
                }
                if (preliminaryPosition > lastPosition) {
                    setSelection(preliminaryPosition);
                    super.layoutChildren();
                }
            }
            
            smoothScrollToPositionFromTop(position, offset);
        }
    }
}